1. Field of the Invention
This invention relates to a laser light generating apparatus in which the laser light from a laser light source is intensified by a optical resonator located outside the laser light source and output after waveform conversion by a non-linear optical crystal element arranged within the optical resonator.
2. Description of the Related Art
As a high-efficiency laser light generating apparatus, there is proposed in e.g., U.S. Pat. No. 5,367,531 such a laser light generating apparatus in which the laser light from a laser light source is intensified by an optical resonator located outside the laser light source and output after wavelength conversion by a non-linear optical crystal element arranged within the light resonator.
With such laser light generating apparatus, it is necessary to control the wavelength of the laser light radiated from the laser light source and the length of an optical path within the light resonator to satisfy the conditions for resonation. It is therefore necessary for the output of the laser light radiated from the laser light source to be kept stable. In general, if there is any return light from the optical resonator to the laser light source, the laser light oscillation tends to be unstable. Thus it is crucial in the above-described laser light generating apparatus to suppress the return light from the light resonator to the laser light source.
Heretofore, an optical isolator is arranged between the laser light source and the light resonator for suppressing the return light to the laser light source by this optical isolator. However, this optical isolator is extremely expensive and prevents the cost reduction of laser light generating apparatus.
As a conventional laser light generating apparatus, such apparatus employing a so called ring type resonator is proposed for an optical resonator for preventing the above-described return light. With this ring type optical resonator, a light resonator 110 is constituted by four reflective mirrors 101, 102, 103 and 104, as shown in FIG. 1, for suppressing the return light to the laser an light source. With such optical resonator 110, the laser light in the laser light source is theoretically only the traveling wave proceeding in the light incident direction of the laser light from the laser light source L1, while the traveling wave proceeding in the opposite direction to the light incident direction of the laser light L2 is not generated such that no laser light is returned to the laser light source. The traveling waves L1 and L2 are referred to hereinafter as the forward traveling wave and the reverse traveling wave, respectively.
With the above-described laser light generating apparatus, it is generally presupposed that the laser light falls vertically on a non-linear optical crystal element used for wavelength conversion. With such a non-linear optical crystal element, an antireflection film is formed on the laser light incident surface for diminishing optical loss. The anti-reflection film should ideally eliminate the reflection completely. However, it is in fact extremely difficult to eliminate refection completely, such that reflection of on the order of 0.1 to 0.5% remains.
Therefore, if a non-linear optical crystal element 111 is arranged within the optical resonator 110 having the first to fourth reflective mirrors 101 to 104, as shown in FIG. 2, the forward traveling wave L1 is reflected by a laser light incident surface of the non-linear optical crystal element 111. The resonant frequency of the reflected light R1 by the non-linear optical crystal element 111 is equal to the resonant frequency of the forward traveling wave L1, due to the reversibility of light, unless a Faraday element or the like is present within the optical resonator 110. Therefore, if there is the slightest residual reflection, the reflected light R1 is in resonation with the forward traveling wave L1, thus generating a reverse traveling wave in the light resonator 110. That is, the reflected light R1 from the non-linear optical crystal element 111 traveling the closed light path in the light resonator 110 in a direction opposite to the forward traveling wave L1 for exciting the reverse traveling wave in the light resonator 110. The reverse traveling wave, thus generated in the inside of the light resonator 110, becomes a mode-matched return light R2 to the laser light source 112, thus making the laser light oscillation unstable.
In such a laser light generating apparatus, employing the ring-type light resonator, return light is produced by reflection from the non-linear optical crystal element, so that an optical isolator needs to be provided between the laser light source and the light resonator, thus making it difficult to lower the production cost.
For preventing reflection from the non-linear optical crystal element, it has also been proposed to cut the surface of the non-linear optical crystal element at a Brewster angle.
However, limitations are imposed on this method since the light handled in non-linear wavelength conversion is not necessarily the light less susceptible to reflection at the Brewster angle.
That is, if type 2 phase matching is used, the input light has different directions of light polarization. Therefore, if reflection of one of the polarized light beams can be prohibited by cutting the surface of the non-linear optical crystal element at the Brewster angle, typically not less than 20% of reflection loss is incurred for the other polarized light.
On the other hand, if the type 1 phase matching is used, the directions of light polarization of two input light beams are the same. Therefore, the reflection loss can be diminished by cutting the surface of the non-linear optical crystal element at the Brewster angle, insofar as these light beams are concerned. However, since the generated third light beam is generally in a direction of polarized light extending normal to the direction of the polarization of the input light, not less than 20% of the reflection loss is incurred insofar as this third light beam is concerned.
The method of cutting the surface of the non-linear optical crystal element at the Brewster angle is not practical since the reflection loss then is increased significantly.
Thus, in the laser light generating apparatus employing the non-linear optical crystal element, the method generally used is to form an anti-reflection film on the surface of the non-linear optical crystal element and to cause the forward traveling wave to be incident perpendicularly on this surface for realizing high efficiency wavelength conversion.
With the conventional laser light generating apparatus, as described above, an optical isolator for suppressing the return light from the optical resonator to the laser light source is indispensable, such that it has been difficult to lower the production cost of the apparatus.